This invention relates mainly to a system for repairing joints in pipes laid underground, such as gas pipes, in their buried state, from their inside, to ensure their sealing.
In pipes laid underground such as gas mains, which consist of lengths of cast iron tubes joined together by pipe joints of the water mains type, the pipe joints are known to become sources of leaks due to changes in mating conditions and hence formation of gaps in the joints caused by the weight of vehicles passing over the pipe, by earthquakes or by aging, or due to deterioration of materials comprising the joints. In order to repair such leaks, and for preventive maintenance against such leaks, it has been proposed in recent years to repair the pipe joints from their inside while the pipe is left underground.
As one form of such repairing systems, a repairing system for joints in pipes laid underground has been proposed, in which the pipe is divided into sections of certain lengths and a pig which is slidable along the inside of the pipe while maintaining sealage at its periphery in contact with the pipe inside is inserted into the section of pipe to be repaired from one of its open ends, with a certain amount of resin supplied in front of the pig. The pig is drawn towards the other open end by a tow cable, filling gaps in pipe joints along the pipe section one after another with resin in the process.
Under the above-mentioned repairing system, it is imperative that care is taken to ensure the entry of resin into gaps in the pipe joints, so that the resin infiltrates and saturates the packing material, if it is to achieve a high sealing effect. Although the resin is able to enter the packing material with relative ease in the case of joints which have developed leaks to achieve a good sealing effect, for pipe joints with no leak, the entry of resin is inhibited by residual air among the packing material. As there are many pipe joints at certain pitch intervals along the pipe, among which those with and without leaks coexist, there remains the problem of the effectiveness of repairing being uneven.
Also under the above-mentioned repairing system, although in the early stages of the repairing operation when there is a large amount of resin ahead of the pig, the viscous drag between the resin and the pipe inside causes a high pressure in the resin which in turn ensures substitution and saturation of gaps in pipe joints with resin, as the amount of resin ahead of the pig diminishes with the progress of the filling process, a decline of pressure in the resin due to viscous drag between the resin and the pipe wall occurs, leading to a drop in filling pressure, the problem of the substitution and saturation process becoming unreliable arising in the later stages of the repairing operation.
This invention seeks to provide an improved repairing system for pipe joints, under which problems such as those mentioned above are avoided.